# How do you do a steam distillation?

Apr 16, 2014

You pass steam through an immiscible organic liquid and collect the distillate.

Procedure:

Many organic liquids with high boiling points decompose when you try to distil them. If a liquid X is immiscible with water, you can do a steam distillation instead.

You pass steam through X in a distillation apparatus and collect the distillate. The distillate contains a layer of water and a layer of X, which you can then separate in a separatory funnel.

Principle:

Because X is immiscible in water, each liquid exerts its own vapour pressure.

${P}_{\text{tot" = P_"liquid" + P_ "water}}$.

The mixture boils and distils when ${P}_{\text{tot" = P_"atm}}$.

Thus, the boiling point of the mixture is less than the boiling point of pure X or pure water. The distillate contains water + X as two immiscible layers.

Example:

The boiling point of bromobenzene is 156 °C, and the boiling point of water is 100 °C.

A mixture of bromobenzene and water boils at 95 °C.

Thus, we can distil bromobenzene at a temperature 61 °C below its normal boiling point.